This invention relates in general to the field of seafloor surveying, and in particular, to apparatus for coupling conventional surface survey systems to the seafloor without loss of accuracy.
The need to accurately locate positions on the seafloor in the nearshore, diver-depth regime is not new. Moorings, piers, pipelines, cables, tracking ranges, oceanographic instrumentation and more recently offshore oil production facilities all require an accurate knowledge of both geographic and relative locations of points on the seafloor. Obtaining this data with high confidence requires three basic steps: determination of the location of a point on the sea surface; establishment of ocean bottom control; and traverse and profile of the seafloor.
Many nearshore survey systems, such as optical theodolites, laser systems, and electronic navigation systems, are available which can determine the locations of a point on the surface of the sea very accurately (within a few inches or feet). However, the process of establishing ocean bottom control, that is, accurately translating a surveyed surface position to a seafloor position, has been the weak link in this technology. There has been no practical way to translate the accurate surface datum to the seafloor even if it were just a few feet below the surface.
Free-fall devices and similar plumb-line techniques although universally used in land surveying do not reliably mark the point directly below except for very shallow water and ideal wave and current conditions. Optical and acoustic techniques have also been used in underwater surveying but these also present problems. The underwater lasers are limited in useful range and have not been developed to accuracies of better than about .+-.2 feet at depths of 60 to 80 feet. The accuracy of conventional sonar systems depends on a knowledge of the local velocity of sound through the water and on the presence of an accurate benchmark. The sonar systems are accurate to only about .+-.1 foot at the depths and ranges of interest in offshore surveying. Fixed rigid towers have been used but these are very clumsy and expensive to build and handle in more than a few feet of water.